This project investigates the immunoregulatory functions of a T-cell factor and a macrophage (M0) factor that are involved in the generation of alloreactive cytotoxic T lymphocytes (CTLs). Both factors are distinct from interleukin-1, interleukin-2 and interferon. The M0 factor appears to provide Lyt 2+ T cells with an activation/differentiation signal which synergizes with IL-2 during CTL development. M0s appear to produce this differentiation signal when induced by a T-cell factor. The T-cell factor coelutes with colony-stimulating factor (CSF) through several steps of purification. Studies are in progress to purify both factors, identify their mechanism of action and determine their relationship to other immunoregulatory molecules.